Vice-presidential candidates come in many shades.
Some have heft. Others are picked for their ability to swing a state. Some should have been running for President. A few should never have been allowed within a million miles of national office.
Rarely, though, does the No 2 define the ticket. That, however, threatens to be the case with Paul Ryan.
He's been the designated running-mate for only a week, and the Republican convention in Tampa, Florida, is still a week away. But already Ryan and his austere economic vision dominate the campaign, overshadowing the stilted and impenetrable Mitt Romney who will actually be Barack Obama's opponent in November.
When he unveiled Ryan last Sunday the clumsy-tongued Romney introduced him as "the next President of the United States". Slips, one is tempted to say, don't get much more Freudian than that.
Romney, the Mormon missionary and businessman who became the successful Republican Governor of a liberal state and seems to change his views as other men change shirts, fits no simple American paradigm. But Paul Ryan does - the wholesome kid from Middle America with an uplifting life story, who conquered adversity and then went on to make very good indeed.
He was born and raised in a middle-class Catholic family in Janesville, southern Wisconsin.
When he was 16, he found his father, a lawyer, dead in his bed of a heart attack. Some children might have been traumatised. Ryan, by all accounts, was galvanised, more than ever certain that people must make their own way in life - a view at the heart of his self-reliant economic creed set to be the central issue of election 2012.
The Ryan of Janesville, where he lives with his wife and three children, was the teenager any family would be proud of. He excelled at high school, worked at McDonald's and loved the outdoor life, in particular deer hunting. To this day, he is a devotee of Wisconsin's four best-known contributions to American happiness: bratwurst, beer, cheese and the Green Bay Packers.
The early deaths of his father, grandfather and great-grandfather also convinced Ryan of the importance of physical fitness. Today virtually everyone who follows the US presidential race knows two things about Paul Ryan. One is that he has a splendidly "ripped" physique, carrying a mere 8 per cent in body fat. The other is that he loves "noodling" - a sport where you jump into a river or lake and catch catfish with your bare hands. "I know it sounds crazy, but it's really exhilarating," Ryan has said.
His conservative economics began to emerge at high school, and crystallised at university. There he became a devotee of the liberal economists Friedrich Hayek and Milton Friedman and a follower of Ayn Rand - the novelist-philosopher who preached individualism and opposed altruism.
These days, Ryan now claims to have abandoned Rand's teachings because of her atheism, but for years he made her seminal book Atlas Shrugged required reading for his Congressional staff.
"Paul was always a politician," a friend from his college days told the New York Times. "He was friendly with everybody. He was into debating the issues, and I always knew he was a conservative Republican."
In 1992, the year he graduated in economics from Miami University in Ohio, Ryan volunteered on his first political campaign (for the local Congressman, now Speaker of the House, John Boehner), and put in a stint in Washington as a staff economist for Wisconsin's Republican senator Bob Kasten. But Kasten lost his seat, and Ryan moved to Empower America, an advocacy group founded by such conservative luminaries as Jeane Kirkpatrick, Ronald Reagan's envoy to the UN, Reagan's one-time Education Secretary Bill Bennett, and Jack Kemp.
If Ryan ever had a substitute father, it was probably Kemp. From Kemp more than anyone, he gained his unflinching belief in supply-side, trickle-down economics, that tax cuts and a slimmed-down government were the key to lasting growth. From his mentor, Ryan acquired not just a political philosophy, but a political style as well. Kemp was a happy warrior, an energetic optimist. Economics may be the dismal science, but like Kemp, Ryan practises it with a smile.
He may not have the genuine sympathy for those who drew life's short straws that made Kemp such an appealing politician. But even Democrats agree Ryan is a nice guy, at least able to discuss the issues in a reasonable fashion. Or, as the New York Times columnist Maureen Dowd, put it, "he's the cutest package that cruelty ever came in".
In Wisconsin, where Ryan in 1998 won the first of seven terms in Congress in the seat covering his native Janesville, he has prevailed in a state that normally votes Democrat. Over the past 13 years, he has emerged as the driving intellectual force of the Republican right.
Nor is his conservatism confined to economics. That fondness for hunting makes him a passionate defender of gun rights. On abortion, Ryan has co-sponsored bills declaring that life begins at conception, that would give fetuses the legal rights of people.
All of which made him a compelling pick for Romney, whose campaign by early August had seemed fatally comatose, conspicuous only for gaffes and an inability to take advantage of the weaknesses of a highly vulnerable incumbent. A jolt of excitement was needed - and Ryan has provided it.
But he's a risk as well. Republicans are an ideological party, ever in search of a master ideologist. Once Newt Gingrich, architect of the 1994 Republican takeover of Congress, filled that role. Now it is Ryan.
There are, alas, two snags. The cuts in Medicare in particular are grist to the Obama mill, as Democrats depict Romney and Ryan as twin faces of a malign Republican Janus, a fusion of vulture capitalist and heartless theorist, bent on making the rich richer on the back of ordinary jobs outsourced to the cheapest country available.
The other snag is that no serious economist has managed to make Ryan's numbers add up. His plans might slash the size of government, but still leave America saddled with massive deficits for decades. Ryan may be driving the Republican ticket, but he may also drive it to its downfall.